Hesitation

OLDSC400

MEMBER
1995 LEXUS SC 400
195,000 MILES

I have a 1995 sc400 with a bad hesitation, has two codes, 52 and 55 both knock sensors, The car hesitates between 1500 and 2600 rpms after 2700 it seems to run fine. The hesitation if at light throdle is so bad the drive train slaps back and forth. I have replace the MAF, TPS, check the fuel pump, ran it in high speed. None of this helps. If I pull the MAF cable off and the ECU is in limp home mode. The car RUN GRET, Also check both coils, seems fine. I have even used a analog scope to check most of the input at the computer, I see the MAF, TPS, O2 values, engine temp, cam signals, gee did I leave anything out. I even check the VF1 and VF2 learn air fuel ratio, both are around 2.1v, Can't sigure this one out, I put new cap's and rotors on last night and will change the plugs tonight to see if this help, but I see no reason why it should, when the car runs great in limp mode. ECU bad?

6 Replies

KHLOW2008

MECHANIC
41,815 POSTS

Have you replaced the knock sensors?

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Tuesday, April 30th, 2013 AT 8:07 PM

OLDSC400

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9 POSTS

No, As I have read that these almost never go bad. Im currently running 100 octang in the car, got rid of some of the hesitation, but still have a lot, just takes longer to set off the knock codes on the ECU. I was thinking about timing? Could the timing be off from a cam senor and then when the ECU is in LIMP mode it runs fine.
This car hesitation is cold or hot. Can a crank or sam sensor fail with out throwing a CODE?

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Thursday, May 2nd, 2013 AT 11:25 AM

KHLOW2008

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Where did you read that they never go bad? In fact they have a very high rate of failure. I have replaced many of them.

Knock sensor fault can affect the performance of the vehicle.

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Thursday, May 2nd, 2013 AT 1:43 PM

OLDSC400

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Many forums talked about there issue with hesitation, many people have tried changing the Knock sensors with no effect. Yes it could be, but for the amont of work to go in and change 2 sensors out, without exhauseting all options would be wrong.

Is there a good way to check the knock sensors on a sc400? They ohm out fine, Now the car will BUCK and hesitate for a long time before the CEL goes on, I wonder if a little knock is causing the hesitation but it takes a BIG knock to set the light?

What about grounding out the knock sensors to stop them from sending any signal to the ECU?

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Thursday, May 2nd, 2013 AT 4:02 PM

KHLOW2008

MECHANIC
41,815 POSTS

When trouble codes are present, you need to rectify it first. The basic procedure is to start with checking wire continuity between the sensor and ECM. Next is visual inspection of the sensor. If the plastic of the knock sensor has signs of overheating, there is no doubt about it being faulty.

If your vehicle had overheated previously, it would definitely kill the knock sensors.

If you want to fry the ECM, then you can try grounding it.

ECM would ignore signals from the knock sensor if it senses it is faulty and would retard ignition timing. If the code is being triggerred, it is not due to other causes but only the knock sensor and/or its circuit.